1. Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
US 2003/0013266 A (corresponding to JP 2003-69012 A) discloses a silicon carbide (SiC) semiconductor device, in which a face A having a (11-20) crystal face orientation is used as a channel. The SiC semiconductor device has a MOS (metal-oxide semiconductor) structure, and a channel mobility is improved by hydrogen anneal or being treated in a wet atmosphere that includes both of a hydrogen (H) atom and an oxygen (O) atom. Specifically, the channel mobility is improved by controlling a concentration or a temperature of the hydrogen anneal or the wet atmosphere.
However, further channel mobility is required for the SiC semiconductor device. US 2007/0045631 A (corresponding to JP 2007-96263 A) by the inventors of the present application discloses that a termination/desorption temperature is determined based on the wet atmosphere or a hydrogen atmosphere. The termination/desorption temperature is a temperature at which a dangling bond between SiC and a gate oxide layer is terminated by an element of hydrogen (H) or hydroxyl (OH), i.e., a temperature at which H or OH desorbs. Specifically, the desorption of H or OH mainly occurs at a temperature about in a range between 800° C. and 900° C., and the termination of the dangling bond by H or OH also occurs in the same temperature range. Therefore, the termination/desorption temperature is about in the range between 800° C. and 900° C. Thus, it is required for keeping the wet atmosphere or the hydrogen atmosphere until the temperature decreases at about 800° C. or lower, preferably at about 700° C. or lower, to terminate the dangling bond by H or OH.
Thus, in a manufacturing method of a SiC semiconductor device having a MOS structure disclosed in US 2007/0045631 A, when a heating process is performed at a temperature over about 800° C. (preferably over about 700° C.) after the gate oxide layer is formed, the heating process is performed in the wet atmosphere or the hydrogen atmosphere. For example, the heating process performed at a temperature over about 800° C. may include a round-off oxidation process for rounding a edge portion of a gate electrode so that an electric field is restricted from concentrating, a first reflow process for forming an interlayer insulation layer after a BPSG (boron phosphorus silicon glass) film is formed, and a second reflow process for rounding an edge portion of a contact hole provided in the interlayer insulation layer so that an electrode member implanted in the contact hole is prevented from cutting off, i.e., a step coverage is improved. Thus, these heating processes are performed in the wet atmosphere or the hydrogen atmosphere.
However, when the heating process is performed in the wet atmosphere, polysilicon that constitutes the gate electrode is oxidized. When an oxidized amount of polysilicon is large, the whole gate electrode may be oxidized and the gate electrode may not function or the gate electrode may not form an ohmic contact with other element.